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當其中一項工作完成時，取消剩餘的工作 (C# 和 Visual Basic)

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藉由使用 Task.WhenAny方法和CancellationToken一起，當一個工作完成後，您可以取消所有剩下的工作。WhenAny 方法會使用工作集合的引數。方法會啟動所有工作並傳回單一工作。表示集合中的所有工作完成時，單一工作完成。

這個範例展示如何使用消除標誌結合WhenAny 來保留第一個工作讓其從工作的集合中結束，並取消剩下的工作。每個工作下載網站的內容。這個範例顯示第下載內容的長度完成並移除其他下載。

注意事項注意事項

若要執行此範例，您必須將 Visual Studio 2012 中， Windows 桌面上的 Visual Studio Express 2012 for或者在您的電腦上安裝 .NET Framework 4.5。

下載範例

您可以下載完成 Windows Presentation Foundation (WPF) 專案從 Async 範例:微調應用程式 中依照下列步驟執行。

  1. 解壓縮您下載的檔案，然後啟動 Visual Studio 2012。

  2. 在功能表列上，選擇 [檔案]、[開啟]、[專案/方案]。

  3. 在 [開啟專案] 對話方塊中，開啟含有範例程式碼會在解壓縮，然後開啟 AsyncFineTuningCS 或 AsyncFineTuningVB 方案的資料夾 (.sln) 檔。

  4. 在 [方案總管]，開啟 [CancelAfterOneTask] 專案的捷徑功能表，然後選取 [設定為啟始專案]。

  5. 選擇 F5 鍵執行專案。

    選取 Ctrl+F5 鍵執行專案，但不偵錯。

  6. 執行程式幾次驗證不同下載先完成。

如果您不要下載專案，您可以檢閱 MainWindow.xaml.vb 或 MainWindow.xaml.cs 檔案在這個主題的結尾。

建置範例

本主題中的範例會加入至 取消一項或多項工作 (C# 和 Visual Basic) 中開發取消工作清單中的專案。這個範例使用同一個使用者介面，雖然 取消 鈕不需要依特定方式使用。

若要建置範例，逐步，請依照 < 下載範例中的」表示部分，不過，選取 [CancelAListOfTasks] 為 [啟始專案]。將在本主題中的變更加入至該專案。

在 [CancelAListOfTasks] 專案的 MainWindow.xaml.vb 或 MainWindow.xaml.cs 檔案中，藉由移動每個網站的處理步驟開始轉換從 AccessTheWebAsync 中的迴圈與下列非同步方法。

' ***Bundle the processing steps for a website into one async method.
Async Function ProcessURLAsync(url As String, client As HttpClient, ct As CancellationToken) As Task(Of Integer)

    ' GetAsync returns a Task(Of HttpResponseMessage). 
    Dim response As HttpResponseMessage = Await client.GetAsync(url, ct)

    ' Retrieve the website contents from the HttpResponseMessage.
    Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync()

    Return urlContents.Length
End Function

// ***Bundle the processing steps for a website into one async method.
async Task<int> ProcessURLAsync(string url, HttpClient client, CancellationToken ct)
{
    // GetAsync returns a Task<HttpResponseMessage>. 
    HttpResponseMessage response = await client.GetAsync(url, ct);

    // Retrieve the website contents from the HttpResponseMessage.
    byte[] urlContents = await response.Content.ReadAsByteArrayAsync();

    return urlContents.Length;
}

在 AccessTheWebAsync中，範例使用查詢、 ToArray<TSource> 方法或 WhenAny 方法建立並啟動工作。WhenAny 應用到陣列上會回傳一個單一的工作，當等待時，會確保工作陣列中的第一個工作會完成。

在 AccessTheWebAsync進行下列變更。星號標示出在程式碼檔案中的變更。

  1. 標記為註解或刪除迴圈。

  2. 建立一個當執行時會產生泛型工作集合的查詢。對 ProcessURLAsync 的每一個呼叫傳回 TResult 是整數的 Task<TResult> 。

    ' ***Create a query that, when executed, returns a collection of tasks.
Dim downloadTasksQuery As IEnumerable(Of Task(Of Integer)) =
    From url In urlList Select ProcessURLAsync(url, client, ct)

    // ***Create a query that, when executed, returns a collection of tasks.
IEnumerable<Task<int>> downloadTasksQuery =
    from url in urlList select ProcessURLAsync(url, client, ct);

  3. 呼叫 ToArray 來執行查詢並啟動工作。在下一個步驟中應用WhenAny 方法將會執行查詢並啟動工作，而不需要使用 ToArray，不過，其他方法可能不是如此。最安全的做法是明確強制查詢的執行。

    ' ***Use ToArray to execute the query and start the download tasks. 
Dim downloadTasks As Task(Of Integer)() = downloadTasksQuery.ToArray()

    // ***Use ToArray to execute the query and start the download tasks. 
Task<int>[] downloadTasks = downloadTasksQuery.ToArray();

  4. 在工作的集合上呼叫 WhenAny 。WhenAny傳回一個Task(Of Task(Of Integer))或Task<Task<int>> 。也就是說 WhenAny 傳回等同於一個單一 Task(Of Integer) 或 Task<int> 的工作，在等候時。這個工作是集合中的第一個完成的工作。您必須先完成的工作指派給 firstFinishedTask。firstFinishedTask 的型別是 Task<TResult>，然而 TResult是一個整數因為那是 ProcessURLAsync 的回傳型別。

    ' ***Call WhenAny and then await the result. The task that finishes 
' first is assigned to firstFinishedTask.
Dim firstFinishedTask As Task(Of Integer) = Await Task.WhenAny(downloadTasks)

    // ***Call WhenAny and then await the result. The task that finishes 
// first is assigned to firstFinishedTask.
Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);

  5. 在此範例中，您只需要專注在先完成的工作上。因此，請使用 CancellationTokenSource.Cancel 來取消剩餘工作。

    ' ***Cancel the rest of the downloads. You just want the first one.
cts.Cancel()

    // ***Cancel the rest of the downloads. You just want the first one.
cts.Cancel();

  6. 最後，請等候 firstFinishedTask 擷取下載內容的長度。

    Dim length = Await firstFinishedTask
resultsTextBox.Text &= String.Format(vbCrLf & "Length of the downloaded website:  {0}" & vbCrLf, length)

    var length = await firstFinishedTask;
resultsTextBox.Text += String.Format("\r\nLength of the downloaded website:  {0}\r\n", length);

執行程式幾次驗證不同下載先完成。

完整範例

下列程式碼完成 MainWindow.xaml.vb 或範例的 MainWindow.xaml.cs 檔案。加入星號標記為這個範例加入的項目。

請注意您必須增加 System.Net.Http的參考。

您可以從 Async範例:微調您的應用程式下載這個專案。

' Add an Imports directive and a reference for System.Net.Http.
Imports System.Net.Http

' Add the following Imports directive for System.Threading.
Imports System.Threading

Class MainWindow

    ' Declare a System.Threading.CancellationTokenSource.
    Dim cts As CancellationTokenSource


    Private Async Sub startButton_Click(sender As Object, e As RoutedEventArgs)

        ' Instantiate the CancellationTokenSource.
        cts = New CancellationTokenSource()

        resultsTextBox.Clear()

        Try
            Await AccessTheWebAsync(cts.Token)
            resultsTextBox.Text &= vbCrLf & "Download complete."

        Catch ex As OperationCanceledException
            resultsTextBox.Text &= vbCrLf & "Download canceled." & vbCrLf

        Catch ex As Exception
            resultsTextBox.Text &= vbCrLf & "Download failed." & vbCrLf
        End Try

        ' Set the CancellationTokenSource to Nothing when the download is complete.
        cts = Nothing
    End Sub


    ' You can still include a Cancel button if you want to.
    Private Sub cancelButton_Click(sender As Object, e As RoutedEventArgs)

        If cts IsNot Nothing Then
            cts.Cancel()
        End If
    End Sub


    ' Provide a parameter for the CancellationToken.
    ' Change the return type to Task because the method has no return statement.
    Async Function AccessTheWebAsync(ct As CancellationToken) As Task

        Dim client As HttpClient = New HttpClient()

        ' Call SetUpURLList to make a list of web addresses.
        Dim urlList As List(Of String) = SetUpURLList()

        '' Comment out or delete the loop.
        ''For Each url In urlList
        ''    ' GetAsync returns a Task(Of HttpResponseMessage). 
        ''    ' Argument ct carries the message if the Cancel button is chosen. 
        ''    ' Note that the Cancel button can cancel all remaining downloads.
        ''    Dim response As HttpResponseMessage = Await client.GetAsync(url, ct)

        ''    ' Retrieve the website contents from the HttpResponseMessage.
        ''    Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync()

        ''    resultsTextBox.Text &=
        ''        String.Format(vbCrLf & "Length of the downloaded string: {0}." & vbCrLf, urlContents.Length)
        ''Next

        ' ***Create a query that, when executed, returns a collection of tasks.
        Dim downloadTasksQuery As IEnumerable(Of Task(Of Integer)) =
            From url In urlList Select ProcessURLAsync(url, client, ct)

        ' ***Use ToArray to execute the query and start the download tasks. 
        Dim downloadTasks As Task(Of Integer)() = downloadTasksQuery.ToArray()

        ' ***Call WhenAny and then await the result. The task that finishes 
        ' first is assigned to firstFinishedTask.
        Dim firstFinishedTask As Task(Of Integer) = Await Task.WhenAny(downloadTasks)

        ' ***Cancel the rest of the downloads. You just want the first one.
        cts.Cancel()

        ' ***Await the first completed task and display the results
        ' Run the program several times to demonstrate that different
        ' websites can finish first.
        Dim length = Await firstFinishedTask
        resultsTextBox.Text &= String.Format(vbCrLf & "Length of the downloaded website:  {0}" & vbCrLf, length)
    End Function


    ' ***Bundle the processing steps for a website into one async method.
    Async Function ProcessURLAsync(url As String, client As HttpClient, ct As CancellationToken) As Task(Of Integer)

        ' GetAsync returns a Task(Of HttpResponseMessage). 
        Dim response As HttpResponseMessage = Await client.GetAsync(url, ct)

        ' Retrieve the website contents from the HttpResponseMessage.
        Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync()

        Return urlContents.Length
    End Function


    ' Add a method that creates a list of web addresses.
    Private Function SetUpURLList() As List(Of String)

        Dim urls = New List(Of String) From
            {
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/en-us/library/hh290138.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
                "https://msdn.microsoft.com/en-us/library/dd470362.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://msdn.microsoft.com/en-us/library/ff730837.aspx"
            }
        Return urls
    End Function

End Class


' Sample output:

' Length of the downloaded website:  158856

' Download complete.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

// Add a using directive and a reference for System.Net.Http.
using System.Net.Http;

// Add the following using directive.
using System.Threading;

namespace CancelAfterOneTask
{
    public partial class MainWindow : Window
    {
        // Declare a System.Threading.CancellationTokenSource.
        CancellationTokenSource cts;

        public MainWindow()
        {
            InitializeComponent();
        }


        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            // Instantiate the CancellationTokenSource.
            cts = new CancellationTokenSource();

            resultsTextBox.Clear();

            try
            {
                await AccessTheWebAsync(cts.Token);
                resultsTextBox.Text += "\r\nDownload complete.";
            }
            catch (OperationCanceledException)
            {
                resultsTextBox.Text += "\r\nDownload canceled.";
            }
            catch (Exception)
            {
                resultsTextBox.Text += "\r\nDownload failed.";
            }

            // Set the CancellationTokenSource to null when the download is complete.
            cts = null;
        }


        // You can still include a Cancel button if you want to.
        private void cancelButton_Click(object sender, RoutedEventArgs e)
        {
            if (cts != null)
            {
                cts.Cancel();
            }
        }


        // Provide a parameter for the CancellationToken.
        async Task AccessTheWebAsync(CancellationToken ct)
        {
            HttpClient client = new HttpClient();

            // Call SetUpURLList to make a list of web addresses.
            List<string> urlList = SetUpURLList();

            // ***Comment out or delete the loop.
            //foreach (var url in urlList)
            //{
            //    // GetAsync returns a Task<HttpResponseMessage>. 
            //    // Argument ct carries the message if the Cancel button is chosen. 
            //    // ***Note that the Cancel button can cancel all remaining downloads.
            //    HttpResponseMessage response = await client.GetAsync(url, ct);

            //    // Retrieve the website contents from the HttpResponseMessage.
            //    byte[] urlContents = await response.Content.ReadAsByteArrayAsync();

            //    resultsTextBox.Text +=
            //        String.Format("\r\nLength of the downloaded string: {0}.\r\n", urlContents.Length);
            //}

            // ***Create a query that, when executed, returns a collection of tasks.
            IEnumerable<Task<int>> downloadTasksQuery =
                from url in urlList select ProcessURLAsync(url, client, ct);

            // ***Use ToArray to execute the query and start the download tasks. 
            Task<int>[] downloadTasks = downloadTasksQuery.ToArray();

            // ***Call WhenAny and then await the result. The task that finishes 
            // first is assigned to firstFinishedTask.
            Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);

            // ***Cancel the rest of the downloads. You just want the first one.
            cts.Cancel();

            // ***Await the first completed task and display the results. 
            // Run the program several times to demonstrate that different
            // websites can finish first.
            var length = await firstFinishedTask;
            resultsTextBox.Text += String.Format("\r\nLength of the downloaded website:  {0}\r\n", length);
        }


        // ***Bundle the processing steps for a website into one async method.
        async Task<int> ProcessURLAsync(string url, HttpClient client, CancellationToken ct)
        {
            // GetAsync returns a Task<HttpResponseMessage>. 
            HttpResponseMessage response = await client.GetAsync(url, ct);

            // Retrieve the website contents from the HttpResponseMessage.
            byte[] urlContents = await response.Content.ReadAsByteArrayAsync();

            return urlContents.Length;
        }


        // Add a method that creates a list of web addresses.
        private List<string> SetUpURLList()
        {
            List<string> urls = new List<string> 
            { 
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/en-us/library/hh290138.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
                "https://msdn.microsoft.com/en-us/library/dd470362.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://msdn.microsoft.com/en-us/library/ff730837.aspx"
            };
            return urls;
        }
    }
    // Sample output:

    // Length of the downloaded website:  158856

    // Download complete.
}

請參閱

參考

WhenAny

概念

微調非同步應用程式 (C# 和 Visual Basic)

使用 Async 和 Await 設計非同步程式 (C# 和 Visual Basic)

其他資源

Async範例:微調您的應用程式